This invention relates generally to the field of magnetic memory disc substrates. In particular, it relates to composite ceramic and glass substrates which, for reasons to be described herein, produce superior substrates for magnetic memory disc products.
The pace of new developments in the computer industry is torrid. Disc recording capacity, for example, continues to increase geometrically, with apparently no end in sight. Higher capacity discs require defect-free, smooth surfaces to allow for error-free operation. Typically, today's high capacity discs spin at 5400 rpm during read/write operations. The next generation will likely spin at 8000 rpm. This means that the pick up head can see velocities of close to 80 mph. At these speeds, even small defects in the surface of the substrate will cause unacceptable errors.
Traditionally, magnetic memory discs have been fabricated of an aluminum metal or alloy substrate with the various magnetic and protective coatings deposited thereon.
This configuration suffers from several inadequacies: chief among these is a lack of stiffness. This problem is only exacerbated by the high speeds demanded of today's discs. In an effort to upgrade the disc substrate material, polycrystalline ceramic materials show great promise. Known for their stiffness and other superior mechanical properties, they lend themselves to this application.
Unfortunately, the potential of these materials remains unrealized. This is due in no small part to the inability of prior workers in the art to fabricate an essentially defect-free coating on the base ceramic substrate material which is both manufacturable and cost effective.
Among the relevant work in the field is:
U.S. Pat. No. 4,408,463 is drawn to a substrate for magnetic disks comprised of a polycrystalline ceramic material coated by a glaze layer; PA1 U.S. Pat. No. 4,816,128 is drawn to a process for making a magnetic recording disk by coating an alumina-based ceramic with glass and then subjecting the coated article to a Hot Isostatic Pressure (HIP) step; PA1 U.S. Pat. No. 4,738,885 is directed to a substrate for a magnetic disc comprising alumina which is HIPped and polished; PA1 U.S. Pat. No. 4,971,932 teaches a magnetic memory storage substrate made of a glass ceramic; PA1 U.S. Pat. No. 4,690,846 is directed to a substrate which comprises alumina coated by glass in a paste or powdered form; PA1 U.S. Pat. No. 4,808,455 is directed to a substrate on which a glass layer is deposited and then subsequently polished; PA1 JP 62-90451 is directed to a substrate of a crystalline material covered by a glass film; and PA1 JP 62-90445 is directed to a substrate of a crystalline material covered by a glass film. PA1 forming a green alumina tape; PA1 firing said tape; and PA1 applying a glass layer to the fired tape prior to cooling.
Defects, in the form of pin holes and seeds, can not be tolerated in a high performance magnetic memory disc substrate. If present, they can wreak havoc on data transfer by causing the head to skip, decreasing the available real estate for recording, and causing problems in coating of the magnetic over layer.
Generally, pin holes and seeds are formed when materials are trapped in, or gas evolves from, the material deposited on the substrate. It is the very nature of the prior art processes which cause these defects. Additional defects to be avoided are inhomogenieties due to lack of mixing, and the like. Since the inventive methods use a molten or pre-melted glass, applicants have solved the problems encountered by the prior art.